At the time of starting an engine, if a relative rotational phase of a driven-side rotating body relative to a drive-side rotating body is set to a most retarded angle phase, with resultant retardation of the closing timing of an intake valve, mixture gas present inside a combustion chamber can flow reversely into an intake pipe, thus leading to disadvantageous reduction in a compression ratio inside the combustion chamber, which invites deterioration of start-up performance. On the other hand, at the time of starting the engine, if the relative rotational phase of the driven-side rotating body relative to the drive-side rotating body is set to a most advanced angle phase, with resultant increase in the valve overlap period, an amount of residual exhaust gas inside the combustion chamber will increase, thus inviting deterioration of start-up performance again.
For this reason, in an attempt to improve the engine start-up performance, there is known a valve opening/closing timing control device configured to restrain the relative rotational phase to an intermediate phase between the most advanced angle phase and the most retarded angle phase (see e.g. Patent Document 1).
An intermediate lock mechanism disclosed in Patent Document 1 includes a locking member and a lock recess into which the locking member engages. And, the locking member includes a first pressure receiving face and a second pressure receiving face, to which an oil pressure for releasing lock is to be applied. Further, the first pressure receiving face is in communication with a retard angle chamber whereas the second pressure receiving face is in communication with an advance angle chamber.
When the pump is stopped after engine stop, oils present inside the advance angle chamber and the retard angle chamber are drained into an oil pan, and also, the oil pressure applied to the locking member is now reduced, thus realizing a locked state. Thereafter, when the engine is started, oil is fed into the advance angle chamber, and when the second pressure receiving face is subjected to a predetermined pressure, the locked state is released, so that an advance angle control is effected. In succession, at time of execution of a retard angle control, oil is fed into the retard angle chamber, so that an oil pressure is applied to the first pressure receiving face, thereby maintaining an unlocked state, in which state the relative rotational phase is changed to a retard angle side.